1. Field of the Invention
The present invention generally relates to a multibit tag useful for object identification having an array of cantilevers, and more particularly to a tag having a novel form of cantilever preferably fabricated from a thin strip of soft magnetic material such as Metglas.TM. or the like preferably separated by a thin space from a small sheet of hard magnetic material.
Further, the present invention relates to tags that use acoustic excitation in conjunction with one or more strips of soft magnetic material, having both ends unclamped or having both ends clamped.
2. Cross-reference to Related Applications
The invention disclosed and claimed herein is related to the inventions disclosed and claimed in co-pending applications Ser. Nos. 08/344,808, 08/344,771, and 08/344,196.
3. Description of the Related Art
For retail tagging, tagging used in the road/air-freight package industry, and pallet tagging in manufacturing processes, a tag is required for identifying a product in detail. With a sufficient number of bits, the tag can be interrogated to determine what the product is, when it was manufactured, its price, whether the product has been properly passed through a check-out counter or kiosk, etc. Tags are also useful in identifying personnel as well as a variety of other animate and inanimate objects.
Thus, tags are useful in retailing, shipping, manufacturing and many other kinds of businesses. A number of different magnetic tag configurations are presently of interest for inventory, theft control and personnel identification. Acoustic excitation is attractive as it tends to be less directional than electromagnetic excitation used conventionally. Many conventional sensors also require power sources as part of the structure and some operate only at low temperatures.
In one conventional tag, a vibration sensor includes one or more cantilevers tuned to resonate at predetermined frequencies. The cantilever vibration causes a gap to be closed which allows current to flow through the cantilever to a microchip and an integrated circuit mounted on the base of the device. Thus, this device requires a power source, wiring, current flow through the device for sensing and an integrated circuit, all of which are part of the tag structure.
Another conventional tag has a plurality of cantilevers, with each cantilever having its own superconducting quantum interference device (SQUID) detector mounted in close proximity to a miniature cantilever. Current must flow through the cantilever. The current supplies the magnetic field which results in a change in flux when the cantilever vibrates. The SQUIDS necessarily make this a low temperature device, i.e. at least as low as liquid nitrogen, though more likely liquid Helium (4 degrees Kelvin). This device does not provide for remote sensing and furthermore requires an onboard power source. Additionally, sensing coils on the device operate only at low temperatures.
Other sensing elements are known in which engine knocking is detected with a feedback mechanism for adjusting the timing to reduce the knocking. Vibrating cantilever elements are used which are either magnetic or piezoelectric. Sensing for the piezoelectric elements is obtained by a current output at the resonant frequency. With the magnetic elements, a change in the reluctance path is detected by a coil wrapped around the core of the magnetic circuit comprising the device. Sensing is part of the device and is not achieved remotely. The excitation is mechanical vibration leading to cantilever resonance since the device is secured to the engine block.
Additionally, a single "bit" resonator is known in which a resonating element is an integral part of the resonator structure, consisting of a magnetostrictive membrane. Magnetic excitation is required, giving rise to an acoustic signal that can be detected remotely.
Thus, the invention provides inexpensive multibit tags different from those which rely on the magnetostrictive effect and are magnetically interrogated. Magnetic tags are generally less expensive than the more conventional radio frequency (RF) tags containing integrated circuits. As mentioned above, the magnetic tags can be used in a variety of different applications, including anti-theft, identification and retail applications.